Na+,K+-ATPase(alsocalledtheNa+,K+-pump orjusttheNa+pump)isfoundinallcellsandisresponsibleforestablishingthegradientsofNa+ andK+ acrosstheplasmamembrane.Thesegradientsinturnprovideenergyforseveralessentialcellfunctions(see ).Na+,K+-ATPaseiscomposedofthreesubunits(α,β,andγ),andtheproteinexistsinthemembranewithastoichiometriccompositionof1α,1β,1γ.Theα subunitcontainsbindingsitesforNa+,K+ andATP.Itisalsothesubunitthatbindscardiacglycosides(e.g.,ouabain),whichspecificallyinhibittheenzyme.Ithasatransmembranedomainandthreeintracellulardomains:phosphorylation(P-domain),nucleotidebinding(N-domain),andactuator(A-domain).Althoughtheα subunitisthefunctionalsubunitoftheenzyme(i.e.,ithydrolyzesATP,bindsNa+ andK+ ,andtranslocatesthemacrossthemembrane),itcannotfunctionwithouttheβ subunit.Theβ subunitisresponsiblefortargetingtheα subunittothemembraneandalsoappearstomodulatetheaffinityoftheNa+,K+-ATPaseforNa+ andK+ .Theα andβ subunitscancarryoutNa+ andK+ transportintheabsenceoftheγ subunit.However,theγ subunitappearstoplayaregulatoryrole.Theγ subunitisamemberofafamilyofproteinscalledFXYD proteins (sonamedfortheFXYDaminoacidsequencefoundintheprotein).
H+-ATPase in the plasma membrane plays an important role in urinary acidification (see
ABC transporters represent a large group of membrane transporters. They are found in both prokaryotic and eukaryotic cells, and they have amino acid domains that bind ATP (i.e., ABC domains). Seven subgroups of ABC transporters in humans and more than 40 specific transporters have been identified to date. They transport a diverse group of molecules/ions, including Cl−, cholesterol, bile acids, drugs, iron, and organic anions.
Because biologically important molecules enter and leave cells through membrane transporters, membrane transport is specific and regulated. Although some membrane transporters are ubiquitously expressed in all cells (e.g., Na+,K+-ATPase), the expression of many other transporters is limited to specific cell types. This specificity of expression tailors the function of the cell to the organ system in which it is located (e.g., the sodium-glucose–linked transporters SGLT-1 and SGLT-2 in the epithelial cells of the intestines and renal proximal tubules). In addition, the amount of a molecule being transported across the membrane can be regulated. Such regulation can take place through altering the number of transporters in the membrane or altering the rate or kinetics of individual transporters (e.g., the time an ion channel stays in the open versus closed state), or both.
Solute and water can be brought into the cell through a process of endocytosis and released from the cell through the process of exocytosis. Endocytosis is the process whereby a piece of the plasma membrane pinches off and